Just by way of an update on this, for a

moment...
Lisa's mum took it in for some new tyres at the end of last week. They found this (the slot/hole in the middle and mangled insulating tape is after they poked about trying to get it out)
So it's had a wheel stud missing/sheared off on the osr wheel by some previous monkey, which had then been covered up by sticking a bottle top (or something) in the hole
Don't know how neither Merc or I failed to notice that when we looked the car over

(looked closely at the brakes and tyres etc.) but I suppose it's not the most 'eye catching' of things and tbh the first things on your mind when looking over a second hand car isn't 'are all the wheel nuts there?)!

I remember now thinking it looked like it had a locking wheel nut on some wheels but not all the others, but didn't think more of it at the time.
Easy enough to fix, just annoying

and shows why the trade continue to have such a bad name...